A storage system is equipped with storage media (HDDs (Hard Disk Drives), SSDs (Solid State Drives) and the like), and a controller for receiving I/O (Input/Output) requests (hereinafter denoted as “I/O requests”) from a host computer and the like. The controller is composed of a CPU, a memory, a storage I/F and a disk I/F, for example, as taught in Patent Literature 1. In the case of a read I/O request, an I/O control program operating in the CPU stores the data read from the storage media via the disk I/F in a cache data area created in the memory. Next, the I/O control program returns the data from the cache data area via the storage I/F to the host computer.